Allow using empty array when indexing on DataStub object (#605)

* Coarse fix for #595

* Add test

* Update test to test different data types

* Make local function getEmptyRepresentation more explicit

* Remove unused code from test

+tests/+unit/dataStubTest.m

@@ -103,3 +103,43 @@ function testObjectCopy(testCase)
 tests.util.verifyContainerEqual(testCase, nwbNew, nwb);
 nwbExport(nwbNew, 'new.nwb');
 end
+
+function testLoadWithEmptyIndices(testCase)
+    nwb = NwbFile(...
+        'identifier', 'DATASTUB',...
+        'session_description', 'test datastub object copy',...
+        'session_start_time', datetime());
+
+    % Add different datatypes to a table, and try to read them in later
+    % using empty indexing on a DataStub representation
+    tableToExport = table( ...
+        {'test'}, ... % Cell
+        0, ... % Double
+        false, ... % Logical
+        struct('x', 1, 'y', 1, 'z', 1) ... % Struct (compound)
+    );
+    dynamicTable = util.table2nwb(tableToExport);
+    nwb.acquisition.set('Test', dynamicTable);
+
+    nwbExport(nwb, 'testLoadWithEmptyIndices.nwb')
+
+    nwbIn = nwbRead('testLoadWithEmptyIndices.nwb', 'ignorecache');
+
+    importedTable = nwbIn.acquisition.get('Test');
+    varNames = transpose( string(importedTable.colnames) );
+
+    for iVarName = varNames
+        iDataStub = importedTable.vectordata.get(iVarName).data;
+
+        testCase.assertClass(iDataStub, 'types.untyped.DataStub')
+        value = iDataStub([]);
+        testCase.assertEmpty(value)
+
+        if isstruct(tableToExport.(iVarName))
+            expectedClass = 'table';
+        else
+            expectedClass = class(tableToExport.(iVarName));
+        end
+        testCase.assertClass(value, expectedClass)
+    end
+end

+types/+untyped/@DataStub/load_mat_style.m

@@ -34,7 +34,14 @@
             , iDimension, dimensionSize);
     end
 
-    if isscalar(userSelection) && ~ischar(userSelection{1})
+    if isscalar(userSelection) && isempty(userSelection{1})
+        % If userselection (indices) is empty, get the first element of this 
+        % DataStub and try to return an empty representation of that type.
+        data = obj.load_mat_style(1);
+        data = getEmptyRepresentation(data);
+        return
+
+    elseif isscalar(userSelection) && ~ischar(userSelection{1})
         % linear index into the fast dimension.
         orderedSelection = unique(userSelection{1});
 
@@ -200,3 +207,23 @@
         indexKeyIndex((indexKeyIndexNextIndex+1):end) = 1;
     end
 end
+
+function emptyInstance = getEmptyRepresentation(nonEmptyInstance)
+    try
+        emptyInstance = nonEmptyInstance;
+        if istable(nonEmptyInstance)
+            % To make an empty table instance, we need to use row/column colon
+            % indices to clear all the table's data. We want to keep the
+            % original table's metadata, like variable names etc, so we clear
+            % the table data instead of creating a new empty table with
+            % table.empty
+            emptyInstance(:, :) = [];
+        else
+            % All other types should support linear indexing.
+            emptyInstance(:) = [];
+        end
+    catch ME
+        error('Failed to retrieve empty type for value of class "%s". Reason:\n%s', ...
+            class(nonEmptyInstance), ME.message)
+    end
+end